This invention relates to a method and apparatus for handoff in a cellular radio communications system. The invention finds particular application in multi-beam, or sectored, cells using spread-spectrum communication.
Cellular radio communications systems are widely used throughout the world to provide telecommunications to mobile users. A geographic area covered by a cellular radio system is divided into cells, each containing cell site equipment at a cell site, through which subscriber units, such as mobile stations, communicate.
In general, an object of cellular radio communications system design is to reduce the number of cell sites required by increasing the range and capacity of the cell site equipment at each one. This is because cell sites are expensive, both because of the equipment required and because of the need for a geographical site. Geographical sites may be costly and may require extensive effort to obtain planning permission. In some areas, suitable sites may even not be available.
The communications ranges in many systems are uplink (mobile to cell site) range limited because of the limited power available at the subscriber unit, which may be a hand-portable subscriber unit. However, any increase in range would mean that fewer cells would be required to cover a given geographical area, thus advantageously reducing the number of cell sites and associated infrastructure costs.
When a cellular radio system is set up in an area of high demand, such as a city, then cell-site communications capacity, rather than range, usually limits cell size. Increased cell site capacity would therefore reduce the required number of cell sites and so reduce costs.
After a cellular radio system has been set up, demand may increase to exceed the capacity of the existing cell sites. A method of upgrading existing cell sites to increase capacity where required might then reduce costs because the capacity of the system could be increased without acquiring any new geographical sites or installing more cell sites.
One approach to increasing range and/or capacity, or to upgrade a cell, is to use directional antennas at a cell site physically to separate radiations at similar frequencies. This is known as sectorisation. It has been proposed to use three-sectored cells, having three antennas with nominally 120xc2x0 azimuthal beamwidth, or hex-sectored cells, having six antennas with nominally 60xc2x0 azimuthal beamwidth (as described for example in U.S. Pat. No. 5,576,717). In each case, one effect of the sectorisation is to reduce interference from mobiles and cell sites in adjacent and nearby cells, and thus to increase the total range and/or capacity of the cell site in a sectored cell relative to a cell using an omni-directional antenna.
In any cellular system, a subscriber unit may move from one cell to another, necessitating transfer of the communication link from one cell site to another by a process known as handoff. In a sectored cell, a subscriber unit may also move from one sector to another, necessitating additional handoffs between the sectors of the cell site.
One mode of communication used in cellular radio systems is spread-spectrum communication, such as code division multiple access (CDMA). In spread-spectrum systems, all cell site transmissions, both in different sectors and in different cells, may be in the same frequency band. A subscriber unit may then communicate with cell sites via more than one sector, in the same or different cells, at the same time. Signals to and from the subscriber unit in different cells, and usually those in different sectors are examples of diversity signals.
To receive diversity signals, a spread-spectrum or CDMA receiver usually comprises a rake receiver consisting of several, such as three or four, parallel correlators (commonly known as fingers). Each diversity signal carries the same information but is differently coded or arrives at the receiver with a different delay, and may be tracked and received independently by one of the fingers of the rake receiver. The combination of the strengths of the diversity signals is then used to improve demodulation of the transmitted data. While there may be fading on each diversity signal, the fades are generally independent. Demodulation based on the combination of several received signals therefore increases reliability. Multipath propagation of a wideband spread-spectrum or CDMA transmission often gives rise to a further plurality of independently receivable signals at a receiver. These multipath diversity signals can be used further to increase reliability of signal reception and decoding.
In a sectored cell, the beams in adjacent sectors must overlap each other to ensure continuity of coverage within the cell. Thus, if the number of sectors increases, then the number of subscriber units within overlapping beam areas may be expected to increase, especially when the wide-angle scattering effects of the channel are taken into consideration. In a spread-spectrum system this can increase the number of subscriber units receiving transmissions in different sectors and would therefore be expected to increase the opportunities for receiving diversity transmissions.
However, a subscriber unit positioned in the overlap between two sectors covered by beams generated by two closely-spaced antennas may experience very little diversity between signals on those beams. This is because the paths of the signals would be very similar, so that the signals cover similar path lengths and are subject to similar fading conditions. Therefore, the signals are likely to be correlated in their fading. This is likely to be a problem particularly in cells having about six or more sectors.
As the number of sectors in a cell increases, it has been proposed to reduce antenna complexity at the cell site by generating beams for more than one sector from a single antenna, such as a phased-array antenna. Although this advantageously simplifies the hardware, or cell site equipment, at the cell site, it also increases the likelihood of correlated fading of beams in adjacent sectors and so would be expected disadvantageously further to reduce the diversity between such beams.
An object of the present invention is to overcome the problem of reduced diversity in sectored cells described above.
A further object of the present invention is to increase the diversity in transmissions used for communication to and from a mobile station in a sectored-cell system.
The invention in its various aspects is defined in the appendent independent claims, to which reference should now be made. Preferred or advantageous features of the invention are set out in dependent subclaims.
The invention is implemented in a cellular radio system which preferably uses spread-spectrum communication, and particularly preferably uses CDMA communication.
In a CDMA system, after a call is set up to or from a subscriber unit, such as a mobile station, via a first sector of a sectored cell, the subscriber unit scans for pilot signals from neighbouring or nearby sectors or from neighbouring or nearby cells (which may be sectored or unsectored). If it finds a pilot signal of sufficient strength it transmits a control signal via the first sector identifying the source of the pilot signal. The system may then initiate a handoff by supporting the call simultaneously via both of the sectors or cells. This process may then be repeated. In this way the subscriber unit may receive transmissions via as many sectors or cells as it has available fingers in its rake receiver.
While receiving two or more transmitted signals at once, a subscriber unit is in handoff. In this state the diversity of the two or more signals may be used to enhance the overall received signal. However, in a sectored cell, downlink (forward link) signals in separate sectors may have a high correlation in their fading, especially if they are derived from a common antenna or closely-spaced antennas at the cell site. Thus, not much diversity gain may be expected to be derived from these signals.
By contrast, signals from different cell sites in different cells certainly will be uncorrelated and so would lead to significant diversity gain. However, a subscriber unit can only be in handoff with a limited number of diversity signals.
In a conventional receiver, diversity signals are selected for combination only on the basis of their signal strength without consideration of their expected or actual degree of correlation, or diversity.
The invention therefore aims to increase diversity benefit during handoff by enabling the subscriber unit, and the cell sites or a base station controller, to select signals for decoding on the basis of the expected diversity of the signals as well as on the basis of signal strengths.
A subscriber unit may be, for example, a mobile station or a fixed wireless access (FWA) station. The following specific embodiments refer mainly to mobile stations but, as the skilled person would appreciate, a number of aspects of the embodiments apply also to FWA.
The meaning of correlation in relation to radio signals is well-known to the skilled person from, for example, the text book xe2x80x9cDigital Communicationsxe2x80x9d, 2nd edition, by John G. Proakis, published by McGraw-Hill Book Company, 1989 (ISBN 0-07-100269-3), which is incorporated herein by reference.